I recent bought an older (1890) house. On the original floors, the wood flooring is sagging between the joists--you can feel the height difference as you walk, up and down, up and down. It's firm, it's just not flat. The joists below are in great condition, huge old timbers, they're just far apart and I'm betting the subfloor is not up to the task of staying flat.

Is this something to be concerned about? What complications might this cause (e.g., could furniture legs break through the floor if between joists)? Or should I ignore it until it comes time to refloor the house? Is it possible to shore this up from underneath?

How far apart are the joists? Do you have access from underneath? And how thick is the flooring?
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The Evil GreeboSep 26 '11 at 15:19

I have access from below (on the first floor anyway). I'm not sure how far apart they are; more than 18", IIRC. No idea on flooring thickness.
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Alex FeinmanSep 26 '11 at 15:20

Is there any bridging between the joists (probably cross bridging in the middle of the span)? With an old house like that you may not even have a subfloor.
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auujaySep 26 '11 at 16:59

1

Could you post a picture of the underside of the floor that shows the joists, particularly where they "connect" with the wall?
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auujaySep 26 '11 at 17:29

Pictures would be great. Also take a straight edge and place it over the floor, measuring the maximum gap. Big warning: if you add joists below, you risk making the hardwood buckle up and squeeeak...
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BryceAug 9 '12 at 21:32

2 Answers
2

Since you can get to the floor from underneath, you can shore up the flooring by adding new joists between the existing ones. This will not be a FUN process, but the process is simple enough, just labor intensive.

First, add cross braces between each joist at each end, of the same dimensions as your existing joists. You'll want them to be snug fitting between each joist, and well supported with joist hangers.

Now, between each brace, halfway between each existing joist, hang new joists to fit between the cross braces you added. Before you permanently hang the joists, prop them up from beneath with 2x4 temporary posts to force the tops up to be level with the tops of the existing joists (pushing the flooring above back into shape), then secure them permanently in place with joist hangers.

Add blocking between the old and new joists as well, to ensure the new joists don't warp/twist.

ok no I don't mean bridging. The new joists have to be supported, so I was describing how to support those joists to transfer load - only I kind of assumed, admittedly, that he can't just hang the joists on the existing beams...
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The Evil GreeboSep 26 '11 at 17:14

1

Yes - thats the 2x4 I mentioned before - make a few temporary jack posts out of 2x4s just to prop the joist up to the proper height before you anchor it in.
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The Evil GreeboSep 26 '11 at 18:29

To make the floor stronger from underneath you have basically two choices.

1) Add new joists between the existing joists, so if they are currently 18" oc they would now be 9"oc (alternating an old joist with a new one you add). This is basically what @The Evil Greebo is referring to.

2) The second option is to sister the existing joists. You glue and screw a new piece of lumber onto the side of each of your existing joists, effectively making them twice as thick. You need to glue and screw along the entire length of the joist because you want it to move with the original joist.

I am not a structural engineer but I think option 1 will actually be a stronger floor.

To answer the other question about furniture legs busting through the floor, yes it can happen if there is no subfloor and the tongue or groove edge of the flooring breaks. This has happened a few times at my house (without a subfloor). Not to imply that anyone has ended up in the basement but the problem areas are if you get the leg of something heavy on just the right spot (a weak board that does not span across two joists) it can crack.

It happened once when someone sat on our sofa and it has also happened with the fridge when moving it around it got on just the wrong spot and broke through. The fix usually involves going into the basement and installing a 2x4 between the joists directly under the cracked board. I just leave the original board in there but now that it is supported underneath it does not move anymore.

I don't think option 2 will do anything to address the problem described. The issue is the floor is sagging between the joists - sistering the joists won't significantly shorten the load points on the flooring enough to eliminate sag.
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The Evil GreeboSep 26 '11 at 17:12

@TheEvilGreebo: You are correct, if the OP is correct in is theory of the problem. However if may actually be that the joists are actually moving a little bit with the live load. Before deciding on a solution, the OP go in the basement while someone else walks on the floor and determine exactly what part is deflecting (the subfloor or the joists).
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auujaySep 26 '11 at 17:28

The floor doesn't deflect when I walk on the joists, so I think it's just the subfloor--or lack thereof!
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Alex FeinmanSep 26 '11 at 17:33

@AlexFeinman: OK, I reread the original question and along with your comment, you're right. Just making the joists stronger will not fix your "sags".
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auujaySep 26 '11 at 17:35